Dense wavelength division multiplexed (DWDM) optical network systems multiplex many wavelengths on one fiber strand and have evolved to be able to support large wavelength counts (e.g., 160 wavelengths) on one fiber strand. Large wavelength counts forced the industry to minimize the spacing between wavelengths and trigger developments in the optical amplifier area, enabling the gain to be flattened so more of the optical spectrum could be used. Additionally, the L band, C band and other regions of this spectrum were defined, and typical solutions use separate amplifiers per band as a compromise between effective impairment compensation and economics.
Until recently, optical networking transport systems had a conventional optical reach up to 800 km. Optical-electrical-optical (OEO) conversions are needed, in the outside plant as regenerators, to extend the reach of these transport systems. OEO conversions also occur at transponders at the point of connection between the transport system and other systems (e.g., cross-connects, SONET multiplexers, etc.). First generation long haul (i.e., optical reach on the order of 500 to 1500 km) and ultra long haul (i.e., optical reach>1500 km) greatly reduce OEO conversions, allowing the transport payload to stay in the optical domain for much longer distances. Next generation transport systems can include photonic cross-connects to allow the wavelengths to be switched from one fiber to another while traveling long distances. Cross-connections can be done manually or via automated optical add-drop multiplexers (OADM) and photonic switches.
In a DWDM system, a group of optical channels (wavelengths) is launched onto a transmission fiber. Periodically, the wavelengths need to be amplified (typically every 80 to 120 Km) to overcome losses in the transmission fiber. This amplification is done using an all-optical amplification means such as an erbium doped fiber amplifier (EDFA). Network elements can also be deployed along the path, such as an optical add/drop multiplexer or a photonic cross-connect, that allow an optical channel to be dropped (terminated) at a local transponder, added (launched) from a local transponder, or photonically switched from one transmission fiber to another. A transponder typically performs an optical-electrical-optical (OEO) conversion function to convert a received WDM wavelength into a client signal (e.g., a SONET signal) and vice versa.
Maintaining the correct target output power per channel is important to the correct operations of a DWDM system. The individual channel power levels needs to be high enough to withstand the loss of optical power over an optical fiber section between the launch point and the next network element, to maintain as large an optical signal to noise ratio (OSNR) as possible. Network elements along the transmission path, such as EDFAs, induce impairments, which add noise to the signal. For example, EDFAs can produce amplifier spontaneous emission (ASE), which degrades the OSNR. If the power per channel becomes too low, the optical signal to noise ratio is decreased, and this may cause an increase in the error rate of the data recovered by the receiver, or perhaps a total loss of the received data. If the power per channel becomes too high, non-linear effects (such as self-phase modulation) may start to manifest themselves due to the behavior of the optical fiber, again causing data with errors at the receiver or a complete loss of the data. A margin has to be allocated for all of the various impairments that can occur along the transmission path of a channel. Proper control of the per-channel power level increases the margin available for other impairments, such as chromatic dispersion, polarization dependent losses, etc. Non-optimal per-channel power level control reduces the available margin for other impairments. Receiver errors occur when the allocated margin is not large enough for the various impairments.
An EDFA cannot control the power of wavelengths individually. Rather, only the overall (total) gain of the amplifier can be controlled. In addition, an EDFA typically produces a non-equal optical gain over a wide range of wavelengths (such as over the C-band or the L-band). EDFAs typically utilize gain flattening filters in order to even out the amplification gain across the amplification band, but there is usually a residual gain inequality (gain tilt) across the amplification band.
DWDM systems may also deploy distributed RAMAN amplification techniques to produce amplification with improved OSNR. RAMAN amplification causes “gain ripples” across the amplification band, i.e., not every optical channel receives exactly the same RAMAN amplification gain.
Other elements along the optical path may also affect the optical power of optical channels in a non-equal manner. For example, optical switching technologies such as one-dimensional MEMS devices, two-dimensional MEMS devices, three-dimensional MEMS devices, and liquid crystal wavelength selective devices can cause non-equal losses of optical power as optical channels are switched from one transmission fiber to another. This is in addition to any tilt that already existed among the channels as they arrive at the optical switch from the various transmission fibers.
Consequently, the target launch power profile of a group of optical channels launched into an optical fiber will move away from the optimal power levels along the optical transmission path. To overcome this, optical channel power level adjustment devices, such as a dynamic gain equalizer (DGEQ), can be deployed at various points in the network to help return the spectral power profile across the transmission band back towards the desired profile. Such devices may be able to adjust the power level of each optical channel individually, or may be able to adjust small groups of channels (e.g. a group of 2 or 4 adjacent channels), or on a sub-channel basis (i.e. adjusting the spectral profile within a channel).
For example, imagine a simple linear chain of amplifiers spaced on average 100 km apart (typical spacing in long haul or ultra-long haul networks). Over a 4000 km optical transmission distance, this would represent approximately 40 amplifiers sites in the transmission path. Spectral control capability (e.g. DGEQ) may be placed, for example, at every second or at every fourth amplifier site, thereby resulting in 10 or 20 spectral control apparatus in the transmission path.
If an event were to happen in the network at or near the beginning of the transmission path, such as an amplifier suddenly causing an abnormal gain tilt (which is a possible failure mode of an EDFA), all the network elements with spectral control capability downstream of that point will want to compensate for the gain tilt.
In an independent control approach, each DGEQ is adjusted independently. In this approach, various control elements may be making the same adjustment in parallel and thus may be conflicting with each other. For example, assume that the power level of channel 1 is too high and has to be brought down. In the independent control approach, many network elements may see that the power level of channel 1 is too high, and in parallel may reduce the power level of this channel (e.g. via a DGEQ). Since the power level is being reduced in parallel, this will make the power level of channel number 1 become lower than the target along the path. Next, the power level will have to be increased, such that the power level may be over-compensated. Using this approach will cause some undershoots and overshoots until the correct steady state condition is reached. The overshoots and undershoots can persist for tens of seconds depending on the update rate of the DGEQ.
Therefore, it is desirable to provide a coordinated method for controlling dynamic gain equalization in an optical transport network.